In search of the secrets of Aging: NIA

Article title: In search of the secrets of Aging: NIA

Conditions: Aging, death

Source: NIA

Is there a maximum biological limit to the human life span, somewhere around 120 years? Or could we live much longer, given the right conditions? Answers to these and other fundamental questions about aging may now be within reach.

One hundred and twenty years, as far as we know, is the longest that anyone has ever lived. A man in Japan, Shirechiyo Izumi, reached the age of 120 years, 237 days in 1986, according to documents that most experts think are authentic. He died after developing pneumonia.

Long lives always make us wonder: What is the secret? Does it lie in the genes? Is it where people live or the way they live -- something they do or do not do? Eat or do not eat? Most of the scientists who study aging, gerontologists, say the secret probably lies in all of the above -- heredity, environment, and lifestyle.

But gerontologists also ask other and more difficult questions. For example, if the 120-year-old had not finally succumbed to illness, could he have lived on and on? Or was he approaching some built-in, biological limit? Is there a maximum human life span beyond which we cannot live no matter how optimal our environment or favorable our genes?

Whether or not there is such a limit, what happens as we age? What are the dynamics of this process and how do they make life spans short, average, or long? Once we understand these dynamics, could they be used to extend everyone's life span to 120 or even, as some scientists speculate, to much greater ages?

And finally for all of us, the most important question: How can insights into longevity be used to fight the diseases and disabilities associated with old age to make sure this period of life is healthy, active, and independent?

In Search of the Secrets of Aging describes what we know so far about the answers to these questions and what we want to know. It gives an overview of research on aging and longevity, showing the major puzzle pieces already in place and, to the extent possible, the shapes of those that are missing.

Life Span and Life Expectancy

This booklet -- and gerontologists -- talk about two kinds of life span. One is maximum life span, the greatest age reached by any member of a species. In humans this is 120 years, we think. The other is average life span, the average age reached by members of a population. Life expectancy, the number of years an individual can expect to live, is based on average life spans.

Average life span and life expectancy in the United States have grown dramatically in this century, from about 47 years in 1900 to about 75 years in 1990. This advance is mostly due to improvements in sanitation, the discovery of antibiotics, and medical care. Now, as scientists make headway against chronic diseases like cancer and heart disease, some think it can be extended even further.

Maximum human life span seems to be another matter. There is no evidence that it has changed for thousands of years despite fabled fountains of youth and biblical tales of long-lived patriarchs. However, very recently, the dream of extending life span has shifted from legend to laboratory. As gerontologists explore the genes, cells, and organs involved in aging, they are uncovering more and more of the secrets of longevity. As a result, life extension may now be more than the stuff of myth and the retardation of disease and disability, realistic goals.

Aging Theories

Gerontology is often described in terms of its major theories. These fall into two main groups, one emphasizing internal biological clocks or "programs," and the other external or environmental forces that damage cells and organs until they can no longer function adequately.

Aging processes can be divided into three general categories -- genetic, biochemical, and physiological. The rest of this booklet describes what we know and don't know in each territory and where we think we are likely to find answers to questions about aging and longevity.

Theories of Aging

Theories of aging fall into two groups. The "programmed" theories hold that aging follows a biological timetable, perhaps a continuation of the one that regulates childhood growth and development. The damage or error theories emphasize environmental assaults to our systems that gradually cause things to go wrong. Many of the theories of aging are not mutually exclusive. Here is a brief and very simplified rundown of the major theories. For more information, see Selected Readings.

Programmed Theories

Programmed Senescence. Aging is the result of the sequential switching on and off of certain genes, with senescence being defined as the time when age-associated deficits are manifested.

Endocrine Theory. Biological clocks act through hormones to control the pace of aging.

Immunological Theory. A programmed decline in immune system functions leads to an increased vulnerability to infectious disease and thus aging and death.

Error Theories

Wear and Tear.Cells and tissues have vital parts that wear out.

Rate of Living. The greater an organism's rate of oxygen basal metabolism, the shorter its life span.

Crosslinking. An accumulation of crosslinked proteins damages cells and tissues, slowing down bodily processes.

Free Radicals.Accumulated damage caused by oxygen radicals causes cells and eventually organs to stop functioning.

Error Catastrophe. Damage to mechanisms that synthesize proteins results in faulty proteins which accumulate to a level that causes catastrophic damage to cells, tissues, and organs.

Somatic Mutation. Genetic mutations occur and accumulate with increasing age, causing cells to deteriorate and malfunction.

Questions: Selected Readings

Finch, C.E., Longevity, Senescence and the Genome, Chicago:University of Chicago Press, 1991.

Institute of Medicine, Extending Life, Enhancing Life: A National Research Agenda on Aging, Washington, DC: National Academy Press, 1992.

Martin, G.R., and Baker, G.T., "Aging and the Aged: Theories of Aging and Life Extension," Encyclopedia of Bioethics, New York: MacMillan, 1993.

Schneider, E.L., and Reed, J.D., "Life Extension," New England Journal of Medicine 313:1159-1168, 1985.

Warner, H., Butler, R.N., Sprott, R.L., Schneider, E.L.,eds., Modern Biological Theories of Aging, New York: Raven, 1987.

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